Beginning with the invention of Morse code and then the telephone, mankind has been seeking new ways to transmit information effectively and efficiently. In this article, Kyle Cassidy explains the popular types of WAN bandwidth delivery used in relaying data from one end to the other.

This article is excerpted from The Concise Guide to Enterprise Internetworking and Security.

"We are in great haste to construct a magnetic telegraph from Maine
to Texas; but Maine and Texas, it might be, have nothing important to communicate…as
if the main object were to talk fast and not to talk sensibly."

—Henry David Thoreau, Walden

Introduction to Bandwidth Delivery: How the Computer
Crashed into the Telephone

There has been an electronic bandwidth infrastructure in this country since
the 1800s, when Samuel Morse's invention allowed messages to be sent long distances
over copper wire nearly instantaneously. Since that time, there has been significant
change in the way data is encoded and transmitted. These changes, and the inherent
differences between voice and data, have created technology that
is often at odds with one or the other. Because voice transmission has long
been the goal of the telephone company (or telco), it is the transmission
of data that usually suffers.

From the beginning, this communications infrastructure was torn between digital
and analog. The initial form of communication over wire, Morse code, was digital
in theory. Binary information was designated by short or long pulses (a dot
being 0, a dash being 1); however, the transmission media was analog. The pulses
were sent as voltage changes along copper line.

Later it was discovered that information could be sent significantly faster
in a wholly analog form, and voice transmission over wire took over. After all,
who wants to tap out Morse code when you can talk instead? Very fast Morse code
operators can send about 30 words a minute—how fast can you talk?

After many people began using voice over wire, the phone company became concerned
that it would eventually run out of copper, or places to hang it. A town with
20 telephone owners could well afford 20 copper wires running from individual
houses to the telco, but a city of 50,000 or half a million certainly could
not.

The goal of transmitting multiple analog signals over a single piece of copper
wire became absolutely necessary for telephone expansion to continue. Eventually,
some clever person discovered frequency-division multiplexing, which
allowed several conversations to be transmitted over a single pair of wires.

How does that work? Well, like many technological miracles, it uses methods
that have existed since the beginning of history. In the same way that you can
tune in to one voice at a cocktail party and carry on a conversation with that
one person while dozens of other people are talking, frequency division multiplexing
allows computers to distinguish among multiple conversations going on at once
by listening only for signals of a higher or lower frequency than the others.

To a certain extent, that's also the way it's done in frequency-division–multiplexed
data signals. Now, you certainly don't want to be in a telephone conversation
where you can hear a half dozen other people talking in the background, so the
signals have to be separated at the telco before they get to you. But how? It
would be extremely difficult for any equipment belonging to the phone company
to figure out what voice belongs with which conversation, so the signal is modified
before it's bound to other signals. This is done by "painting" the
signal to change its characteristics.

For example, you have the following four streams of numbers you want to send
along the same wire:

6

12

9

3

8

9

12

21

6

6

14

9

17

11

3

7

If you just toss them on the wire, it will be impossible to sort them out because
the numbers are all very similar. However, if you add 100 to each number in
column 2, add 200 to every number in column 3, and add 300 to every number in
column 4, you get numbers that are easy to separate at the receiving end:

6

112

209

303

8

109

212

321

6

106

214

309

17

111

203

307

So, by adding a known quantity to the frequency of the signal, they're made
unique. They're decoded at the telco, and the proper conversation is sent to
your house along your dedicated phone circuit. Figure
1 shows how it might look as sound waves.

Figure 1
Frequency-division multiplexing allows multiple signals to travel over a single
wire.

Packet-Switched Versus Circuit-Switched Networks

Data transmission can be either packet-switched or circuit-switched. There
are advantages to each.

Packet-Switched

Large blocks of data are broken down into smaller parts, called packets.
Each of these packets contains all the addressing information to get it from
point A to point B. The packets take whatever route is available; there is no
fixed path.

Circuit-Switched

A path from point A to point B is negotiated at the beginning of the transmission
and kept open until the end of the transmission. During that time, nothing else
can use that pipe.

The best example of circuit-switched networking is a telephone call. You dial
your best friend, and a circuit is opened between your phone and hers. As long
as you have the phones off the hook, that data pipe belongs to you. No one else
can use it. If someone calls your house, he'll get a busy signal. A circuit-switched
network connection is the fastest and most reliable, but it suffers because
no one else can use the line. If you were to take a one-hour phone conversation
and remove all the fraction of seconds where there is silence on the line, you'll
be left with a lot of silence. Every time you're transmitting silence, you're
wasting bandwidth. Unlike people, computers talk only when they have something
to say, so their conversations are much more direct—hence, packet-switched.

The Telco Engineers Versus the Network Engineers

When computers crashed into the phone company, there was some carnage. From
the outset, telco engineers and computer network engineers were at design odds
with one another. After all, they have entirely different goals. The primary
goal of the telco is to deliver real-time voice transmission. This can be done
at the expense of cost and accuracy. Cost is passed on down the line to the
consumer, and accuracy…well, the assumption is that if there's a distracting
noise in the background, people will retransmit themselves, usually in the manner
of, "Huh? What did you say Bob? There was a bus passing by just then."
Network engineers are more concerned that data will be accurate and less concerned
that it will be real-time. If faulty wiring causes a handful of static to be
thrown into a telephone conversation, most people won't notice, but that same
static could alter critical data in the transmission of a computer file, resulting
in errors.

What we are left with is a telephone infrastructure designed for analog voice
through which digital computer data must be sent. There is a push today for
an entirely digital telco that can serve both voice and data. To an extent,
this is already happening. Much of the innards of the U.S. telephone system
is digital; what remains is known as the "last copper mile," or the
local loop of wiring between a consumer's house and the telco building.
The investment in that wiring and telephones compatible with it is extensive
and expensive, and its replacement will be hard-fought.

For this reason, we have an amalgamation of technologies providing service
in sometimes rather strange and kludgy ways.